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A Review on the Rehabilitation Exoskeletons for the Lower Limbs of the Elderly and the Disabled

Electronics 2022, 11(3), 388; https://doi.org/10.3390/electronics11030388

by Tao Wang^1,†, Bin Zhang^2,†, Chenhao Liu², Tao Liu²

, Yi Han³, Shuoyu Wang³, João P. Ferreira⁴

, Wei Dong^5,* and Xiufeng Zhang^6,*

Reviewer 1: Anonymous

Reviewer 2: Anonymous

Reviewer 3: Anonymous

Electronics 2022, 11(3), 388; https://doi.org/10.3390/electronics11030388

Submission received: 11 November 2021 / Revised: 16 January 2022 / Accepted: 19 January 2022 / Published: 27 January 2022

(This article belongs to the Special Issue Physical Diagnosis and Rehabilitation Technologies)

Round 1

Reviewer 1 Report

As far as I could tell, this is a brief presentation of the work done in this field.

I would have appreciated some more technical descriptions of the presented solutions. I find the paper rather poor in covering technical data.

Author Response

We appreciate the reviewer’s valuable suggestion and we accept the comment. We added some more technical descriptions of the presented solutions. Please refer to the highlighted parts in Abstract, Section I and Table I in Section II.

“The exoskeleton system is a wearable human-machine integrated mechanical device. In re-cent years, the vigorous development of exoskeletal technology has brought new ideas to the rehabilitation and medical treatment of patients with motion dysfunction, which is expected to help such people complete their daily physiological activities, or even reshape their motion function. The medical exoskeletons conduct assistance based on detecting intention, control algorithm, and high-performance actuators. In this paper, we review the medical exoskeletons from the aspects of overall design, drive unit, intention perception, compliant control and efficiency evaluation. We discussed the complexity and coupling of human-machine integration system, and the high expectations for safety and comfort, the obstacles in application.”

“The research work of exoskeleton technology is of great significance, and the de-sign and development of exoskeleton that helps the elderly and the disabled is the general trend in the current medical rehabilitation situation. In this paper, we reviewed the medical exoskeletons from the aspects of overall design, drive unit, intention perception, compliant control and efficiency evaluation. We discussed the complexity and coupling of human-machine integration system, and the high expectations for safety and com-fort, the obstacles in application and we hope this paper can provide a over-all guideline to design a rehabilitation exoskeleton system.”

Table I. Summary of Existing Exoskeletons and Their Related Technical Details

Devices	Mechanical Type	Actuator	Control Strategies	Target Parts of Human	Validation Methods
Lokomat [15]	Treadmill based exoskeleton	Motor	Position and impedance	Hip and knee	EMG activity
ALEX III [16]	Overground	Motor	Balance and impedance	Hip, knee and ankle	Reshape walking ability
ReWalk [17]	Overground	Motor	Force and impedance	Hip and knee	Walking assistance
Indergo [21]	Overground	Motor	Position and force control	Hip and knee	Walking assistance
Mindwalker [24]	Overground	SEA-motor	EEG-based position control	Hip, knee and ankle	Reshape walking ability
Ankle-foot Exoskeleton [31]	Overground	Pneumatic Muscle	Position and force control	Ankle	Reduction in Metabolic (21%)
Rex [37]	Overground	Motor	Balance and force control	Hip, knee and ankle	Reshape walking ability
HAL [38]	Overground	Motor	EMG-based force control	Hip, knee and ankle	Walking assistance

“As shown in Table I, motors were the common actuators to drive the motions of exoskeletons which were easily controlled based on the developed algorithms. The devices were designed based on the level of losing mobility. Most of exoskeletons were target to the hip joints and could assistant the patients walk again based on balance control. Some exoskeletons introduced crutches to avoid tumble. However, there is still lacking of standard validation methods to confirm the effectiveness of the exoskeletons. Some systems drive a single joint such as knee and ankle, and the wearer often participate in fixed posture to complete daily physical activities such as walking.”

Author Response File: Author Response.docx

Reviewer 2 Report

Here are my suggestions to improve the paper:

Lines 37-41 (until “…family.”): Add a reference

In section 2.1, you talk about how an exoskeleton is made showing pictures of real systems. It should be interesting to see also a schematic diagram of the generic system identifying the main blocks of it.

Add tables in the paper to compare the different medical devices manufactured by different companies. You should make comparison of a few technical aspects too, using numbers if you can. The tables must summarize the qualitative and quantitative features that you discuss in the paper. Moreover, a table of advantages and drawbacks from a patient perspective might be also useful.

In the paper there are many typos, repeated words, lack of punctuation, etc. Please, revise all the text very carefully.

Author Response

Response to Reviewer 2 Comments

Here are my suggestions to improve the paper:

Point 1: Lines 37-41 (until “…family.”): Add a reference

Response 1: Thank you for your advice. We added a reference about the background of aging population.

“Xie, XY., Lin, Gz., Huang, Q. et al. Opinions and clinical practice of functional movement disorders: a nationwide survey of clinicians in China. BMC Neurol. 2021, 21, 435.”

Please refer to the No. 1 reference.

Point 2: In section 2.1, you talk about how an exoskeleton is made showing pictures of real systems. It should be interesting to see also a schematic diagram of the generic system identifying the main blocks of it.

Response 2: The reviewer pointed that a schematic diagram would make it more interesting to show the main parts of the exoskeletons. We appreciate the reviewer’s valuable suggestion and added a schematic diagram to describe the structure of exoskeletons based on the surveyed literature.

Figure 2. Overall exoskeleton plan for helping the elderly and the disabled. (a) Schematic diagram to describe the structure of exoskeletons. (b) Based on crutches [17]; (c) Based on the platform [15]; (d) Based on the self-balance design [16].

Point 3: Add tables in the paper to compare the different medical devices manufactured by different companies. You should make comparison of a few technical aspects too, using numbers if you can. The tables must summarize the qualitative and quantitative features that you discuss in the paper. Moreover, a table of advantages and drawbacks from a patient perspective might be also useful.

Response 3: Thank you again for your comment. We added a table to compare the differences of the proposed medical exoskeletons.

Table I. Summary of Existing Exoskeletons and Their Related Technical Details

Devices	Mechanical Type	Actuator	Control Strategies	Target Parts of Human	Validation Methods
Lokomat [15]	Treadmill based exoskeleton	Motor	Position and impedance	Hip and knee	EMG activity
ALEX III [16]	Overground	Motor	Balance and impedance	Hip, knee and ankle	Reshape walking ability
ReWalk [17]	Overground	Motor	Force and impedance	Hip and knee	Walking assistance
Indergo [21]	Overground	Motor	Position and force control	Hip and knee	Walking assistance
Mindwalker [24]	Overground	SEA-motor	EEG-based position control	Hip, knee and ankle	Reshape walking ability
Ankle-foot Exoskeleton [31]	Overground	Pneumatic Muscle	Position and force control	Ankle	Reduction in Metabolic (21%)
Rex [37]	Overground	Motor	Balance and force control	Hip, knee and ankle	Reshape walking ability
HAL [38]	Overground	Motor	EMG-based force control	Hip, knee and ankle	Walking assistance

Point 4: In the paper there are many typos, repeated words, lack of punctuation, etc. Please, revise all the text very carefully.

Response 4: We appreciate the reviewer’s critical comment. We revised the manuscript and correct the grammar typos.

Line 60: "to function simple and inconvenient to use". We replaced line 60 with “… but such devices are unintelligent and inconvenient to use”.

Line 84: "With the help of flexible control algorithm". And the sentence was replaced by “The control algorithm generated the instructions set to maintain human-machine motor coordination, …”.

Line 87: "save manpower" "time costs" "indispensable backbone". “…but also greatly save the cost of manpower and time and will play an important role in medical service system”.

Line 92: We revised this paragraph to “To sum it up, the research work of exoskeleton technology is of great significance, and the design and development of exoskeleton that helps the elderly and the disabled is the general trend in the current medical rehabilitation situation”.

Line 119: "with part or completely lose mobility". We corrected this phrase to “… the disabled who have lost their mobility”.

Line 120:"worn for normal human people". The inappropriate phrase are replaced by “… worn for people with normal mobility”.

Line 158: "... but often make bloated structure and poor control performance". We revised this sentence to “… the structure was bulky and the control performance was poor”.

Line 160:"pneumatic muscle". We corrected this phrase to “pneumatic artificial muscle”.

Line 197:"A single sensing data is one-sided in responding ...". The incomprehensible sentence was replaced by “A single sensing data can only reflect limited information of the human-computer interaction process …”.

Line 200: We revised this misleading sentence to “For data fusion algorithms, according to the difference of data processing levels, it can be divided into data level, feature level and decision level fusion”.

Line 214: We revised those incomprehensible paragraphs as shown in follows.

“In the gait trajectory planning algorithm, the exoskeleton joints are controlled to produce periodic motions according to the pre-designed path, which simulates the gait of humans completing daily activities, so as to achieve the coordination of human and machine actions. The reference movement trajectory can be set in a variety of ways. Firstly, the gait trajectory playback strategy that directly uses healthy human gait parameters is widely used in rehabilitation medical exoskeleton. For instance, systems such as WPAL [40, 41] in Japan and Mina [42, 43] in the United States can collect gait data of normal wearers and reproduce the movement process when assisting patients Secondly, a mathematical model-based gait trajectory generation method can also be employed to calculate the required motion parameters through related theories. For example, Rex in New Zealand generates a gait based on the ZMP model, so that the system has self-balancing ability. And ATLAS in Spanish plans the corresponding motion trajectory based on the inverted pendulum model, and calculates the key parameters such as step length and step height required to complete the desired gait [44].

The impedance/admittance control algorithm establishes the impedance/admittance model of the man-machine coupling system to realize the mixed control of the force and position of the joint motion. McGill University in Canada designed a new adaptive impedance control strategy, which combines backstepping control, time delay estimation and interference observer, which effectively improves the effect of passive assisted rehabilitation training [45]. The University of Twente in the Netherlands has designed and verified the admittance controller on the LOPES II exoskeleton, which indicates that this method has a good auxiliary effect on people with motor dysfunction [46].

The control algorithm based on biological information analyses the control input required for the movement of the exoskeleton on the basis of collecting and analyzing the bioelectric signal of the human body. The Italian Institute of Technology combines the Hill muscle model and sEMG signals to estimate the driving torque required for the knee joint to complete the exercise in real time [47]. The University of Michigan in the United States uses an adaptive gain proportional EMG controller for ankle joint assistance, relying on dynamic gain to map the wearer’s muscle activity to actuation control signals, and trials showed that the system significantly enhanced ankle strength and reduced metabolic consumption [48].”

Line 253: "the strategy of heart rate". The inappropriate expression was corrected to “the strategies of online detection of physiological data such as heart rate and oxygen consumption …”.

Line 264: "human-machine incompatibility", This misleading sentence was replaced by “The suitability of the structure of exoskeleton and human body is significantly low”.

Line 271: "inaccuracy of human intention acquisition and prediction". We revised this misleading sentence to “The recognition and prediction of human movement intention is inaccurate”.

Point 5: Give some more technical overview in the abstract.  

Response 5: We appreciate the reviewer’s critical comment about the abstract and we re-write the abstract.

Point 6: The introduction can be improved and show your contribution at the end of the introduction.

Response 6: We appreciate the reviewer’s valuable suggestion. We revised the last paragraph of Section I to make it clear to describe the structure and the contribution of this manuscript.

Author Response File: Author Response.docx

Reviewer 3 Report

The manuscript presents a variety of work, related to the use of exoskeletons in rehabilitation and healthcare. The manuscript is interesting to read but is rather generic regarding the presented subjects, and could be considered as a popular science presentation of the subject, reviewing several subjects. The manuscript outlines four open research areas. However, these areas are identified by generic considerations and common sense, rather than analysing the literature. There is little new knowledge (e.g., structuring existing literature, analysis of research gaps) that can be learnt from this manuscript.

The presented literature is relevant. However, it is unclear how this literature is retrieved. Thus, it is not possible to say anything about completeness. Further, it is difficult to consider whether the identified research areas in Section 3 really are not covered yet. For the presented research, it is only described what their respective contribution is, but not which research areas are not covered.

Related to this, several systems are listed in the manuscript; these systems are already implemented and in use, but the authors claim that there are still research gaps. Whether there really a research gap or whether the used methodology is satisfactory cannot be considered out of the content of the manuscript. The following question remains: Given that there is already a variety of research and systems, what of these are not satisfactory. The manuscript does not give a qualified answer on this.

There is a large number of incomprehensible sentences. On some occasions, it is impossible to guess what the authors could have meant. Some examples of such examples are outlined in the detailed comments.

Detailed comments:

Line 60: incomprehensible. what do you mean by "to function simple and inconvenient to use"? Usually, the simple use of an interface is a good thing. However, I suspect that you might mean something else here, as the combination with "inconvenient" does not fit.

Line 84: incomprehensible: "With the help of flexible control algorithm"

Line 87ff: incomprehensible sentence. further: Line 90: what do you mean by "save manpower"? "time costs"? "indispensable backbone"?

Line 92: Although it might be true that exoskeletons are of great significance, these three lines are not supported by the literature presented so far.

Line 97: The history of exoskeletons in rehabilitation starts probably much earlier. Further, diverse mechanical aids for the disabled and elderly could also be considered as exoskeletons.

Line 102ff: These lines are about five-year plans, but Reference [12] points to an excellent review article about soft exoskeletons. Please refer to these plans. Please refer also to Reference [12] and present the findings of this review. There are also further review articles, which input should be presented; your review article should build on these.

Line 119: incomprehensible. "with part or completely lose mobility".

Line 120: incomprehensible: "worn for normal human people". What do you mean by "normal"? and why "human people" ?

Line 158: incomprehensible "... but often make bloated structure and poor control performance".

Line 160: incomprehensible: "pneumatic muscle"

Line 197: incomprehensible. "A single sensing data is one-sided in responding ...".

Line 200: incomprehensible. This sentence has no structure. Do you refer to a layered structure? Is there a reference for a layered structure that is used for exoskeletons? The entire paragraph is incomprehensible. Is there literature that supports this?

Line 214 ff: paragraph is incomprehensible. Also, further paragraphs are difficult to understand.

Line 253: incomprehensible: what is "the strategy of heart rate"?

Line 264: Incomprehensible. what is "human-machine incompatibility"? Why "overall structure".

Line 271: incomprehensible. what is "inaccuracy of human intention acquisition and prediction"? Why is it obvious? How do systems that currently are developed solve this?

Line 279: Incomprehensible: what is the lightweight and high-to-mass ratio? This does not make sense. Also: what are "power-to-mass ratios"?

Line 284; Incomprehensible: what is incoordination?

The four "technical bottlenecks" are very generic, and there might be many different solutions dependent on the application area. Further, for some application areas (e.g., stroke) there are cognitive issues in the patient that must be addressed, while for other patients, e.g., rehabilitation after injuries, the mechanic issues are important. These are different research areas, and the review should be structured accordingly.

Please also identify in your review which research areas (and research gaps in the research areas) are present; i.e., are the research gaps in algorithms, sensor development, computer science, pneumatics, mechanical engineering, electrical engineering, etc. Please also refer to literature in these areas that support this.

Author Response

Response to Reviewer 3 Comments

Point 1: The manuscript presents a variety of work, related to the use of exoskeletons in rehabilitation and healthcare. The manuscript is interesting to read but is rather generic regarding the presented subjects, and could be considered as a popular science presentation of the subject, reviewing several subjects. The manuscript outlines four open research areas. However, these areas are identified by generic considerations and common sense, rather than analysing the literature. There is little new knowledge (e.g., structuring existing literature, analysis of research gaps) that can be learnt from this manuscript.

Response 1: We appreciate the reviewer’s critical comment about the vague contribution of this paper. We added statements to clarify the contribution of this paper in the last paragraph, Section I.

“The research work of exoskeleton technology is of great significance, and the de-sign and development of exoskeleton that helps the elderly and the disabled is the general trend in the current medical rehabilitation situation. In this paper, we reviewed the medical exoskeletons from the aspects of overall design, drive unit, intention perception, compliant control and efficiency evaluation. We discussed the complexity and coupling of human-machine integration system, and the high expectations for safety and com-fort, the obstacles in application and we hope this paper can provide an overall guideline to design a rehabilitation exoskeleton system.”

Point 2: The presented literature is relevant. However, it is unclear how this literature is retrieved. Thus, it is not possible to say anything about completeness. Further, it is difficult to consider whether the identified research areas in Section 3 really are not covered yet. For the presented research, it is only described what their respective contribution is, but not which research areas are not covered.

Response 2: Thank you very much for your comments. We added a table to compare the differences of the proposed medical exoskeletons.

Table I. Summary of Existing Exoskeletons and Their Related Technical Details

Devices	Mechanical Type	Actuator	Control Strategies	Target Parts of Human	Validation Methods
Lokomat [15]	Treadmill based exoskeleton	Motor	Position and impedance	Hip and knee	EMG activity
ALEX III [16]	Overground	Motor	Balance and impedance	Hip, knee and ankle	Reshape walking ability
ReWalk [17]	Overground	Motor	Force and impedance	Hip and knee	Walking assistance
Indergo [21]	Overground	Motor	Position and force control	Hip and knee	Walking assistance
Mindwalker [24]	Overground	SEA-motor	EEG-based position control	Hip, knee and ankle	Reshape walking ability
Ankle-foot Exoskeleton [31]	Overground	Pneumatic Muscle	Position and force control	Ankle	Reduction in Metabolic (21%)
Rex [37]	Overground	Motor	Balance and force control	Hip, knee and ankle	Reshape walking ability
HAL [38]	Overground	Motor	EMG-based force control	Hip, knee and ankle	Walking assistance

Point 3: Related to this, several systems are listed in the manuscript; these systems are already implemented and in use, but the authors claim that there are still research gaps. Whether there really a research gap or whether the used methodology is satisfactory cannot be considered out of the content of the manuscript. The following question remains: Given that there is already a variety of research and systems, what of these are not satisfactory. The manuscript does not give a qualified answer on this.

Response 3: Thank you very much for your comments. We listed a table to compare the differences of the rehabilitation exoskeletons from the aspects of mechanical type, type of actuator, control strategy, target parts of human, and validation methods.

Detailed comments:

Point 4: Line 60: incomprehensible. what do you mean by "to function simple and inconvenient to use"? Usually, the simple use of an interface is a good thing. However, I suspect that you might mean something else here, as the combination with "inconvenient" does not fit.

Response 4: Thank you very much for your comments. I understand that your concern is using the word “simple” and “inconvenient” is contradictory. Actually, by “simple” I mean “primitive and unintelligent”. We replaced line 60 with “… but such devices are unintelligent and inconvenient to use”.

Point 5: Line 84: incomprehensible: "With the help of flexible control algorithm"

Response 5: We appreciate for your comments. This sentence is a grammatical error due to our negligence. And the sentence was replaced by “The control algorithm generated the instructions set to maintain human-machine motor coordination, …”.

Point 6: Line 87: incomprehensible sentence. further: Line 90: what do you mean by "save manpower"? "time costs"? "indispensable backbone"?

Response 6: Thank you very much for your critical comments. We revised this sentence to “…but also greatly save the cost of manpower and time and will play an important role in medical service system”.

Point 7: Line 92: Although it might be true that exoskeletons are of great significance, these three lines are not supported by the literature presented so far.

Response 7: We appreciate for your comments. I understand that the meaning expressed by this paragraph is too exaggerated. We revised this paragraph to “To sum it up, the research work of exoskeleton technology is of great significance, and the design and development of exoskeleton that helps the elderly and the disabled is the general trend in the current medical rehabilitation situation”.

Point 8: Line 97: The history of exoskeletons in rehabilitation starts probably much earlier. Further, diverse mechanical aids for the disabled and elderly could also be considered as exoskeletons. Line 102ff: These lines are about five-year plans, but Reference [12] points to an excellent review article about soft exoskeletons. Please refer to these plans. Please refer also to Reference [12] and present the findings of this review. There are also further review articles, which input should be presented; your review article should build on these.

Response 8: Thank you very much for your comments. I understand that your concern is how the system reflects its monitoring function (or mechanical failure in the mechanism). The system is oriented to clinical application. Firstly, we add a pressure sensor to the turning mattress to control the inflation of the air pump by setting the limit of the pressure value.

Point 9: Line 119: incomprehensible. "with part or completely lose mobility".

Response 9: Thank you very much for your comments. We corrected this phrase to “… the disabled who have lost their mobility”.

Point 10: Line 120: incomprehensible: "worn for normal human people". What do you mean by "normal"? and why "human people" ?

Response 10: Thank you very much for your critical comments. The inappropriate phrase are replaced by “… worn for people with normal mobility”.

Point 11: Line 158: incomprehensible "... but often make bloated structure and poor control performance".

Response 11: Thank you very much for your comments. We revised this sentence to “… the structure was bulky and the control performance was poor”.

Point 12: Line 160: incomprehensible: "pneumatic muscle"

Response 12: Thank you very much for your critical comments. We corrected this phrase to “pneumatic artificial muscle”.

Point 13: Line 197: incomprehensible. "A single sensing data is one-sided in responding ...".

Response 13: We appreciate for your comments. The incomprehensible sentence was replaced by “A single sensing data can only reflect limited information of the human-computer interaction process …”.

Point 14: Line 200: incomprehensible. This sentence has no structure. Do you refer to a layered structure? Is there a reference for a layered structure that is used for exoskeletons? The entire paragraph is incomprehensible. Is there literature that supports this?

Response 14: Thank you very much for your critical comments. We revised this misleading sentence to “For data fusion algorithms, according to the difference of data processing levels, it can be divided into data level, feature level and decision level fusion”.

Point 15: Line 214 ff: paragraph is incomprehensible. Also, further paragraphs are difficult to understand.

Response 15: We appreciate for your comments. We revised those incomprehensible paragraphs as shown in follows.

Point 16: Line 253: incomprehensible: what is "the strategy of heart rate"?

Response 16: Thank you very much for your critical comments. The inappropriate expression was corrected to “the strategies of online detection of physiological data such as heart rate and oxygen consumption …”.

Point 17: Line 264: Incomprehensible. what is "human-machine incompatibility"? Why "overall structure".

Response 17: Thank you very much for your comments. This misleading sentence was replaced by “The suitability of the structure of exoskeleton and human body is significantly low”.

Point 18: Line 271: incomprehensible. what is "inaccuracy of human intention acquisition and prediction"? Why is it obvious? How do systems that currently are developed solve this?

Response 18: We appreciate for your comments. We revised this misleading sentence to “The recognition and prediction of human movement intention is inaccurate”.

Point 19: Line 279: Incomprehensible: what is the lightweight and high-to-mass ratio? This does not make sense. Also: what are "power-to-mass ratios"?

Response 19: Thank you very much for your comments. We corrected these inappropriate phrases and what we wanted to express was “power-weight ratio”. And the whole paragraph was revised to “Lightweight and high power-weight ratio of driving units are difficult to achieve. Existing exoskeleton drive units have large volume and mass, and often lower power-weight ratios, resulting in large and bloated overall structural forms. Therefore, it is necessary to explore more effective driving forms as well as innovative design and optimization methods of high power density driving units”.

Point 20: Line 284; Incomprehensible: what is incoordination?

Response 20: Thank you very much for your critical comments. We revised this incomprehensible sentence to “The inconsistency between system motion control and human motion is prominent”.

Point 21: The four "technical bottlenecks" are very generic, and there might be many different solutions dependent on the application area. Further, for some application areas (e.g., stroke) there are cognitive issues in the patient that must be addressed, while for other patients, e.g., rehabilitation after injuries, the mechanic issues are important. These are different research areas, and the review should be structured accordingly.

Response 21: Thank you very much for your comments. We listed a table to compare the differences of the rehabilitation exoskeletons from the aspects of mechanical type, type of actuator, control strategy, target parts of human, and validation methods.

Point 22: Please also identify in your review which research areas (and research gaps in the research areas) are present; i.e., are the research gaps in algorithms, sensor development, computer science, pneumatics, mechanical engineering, electrical engineering, etc. Please also refer to literature in these areas that support this.

Response 22: We appreciate the reviewer’s valuable comment and we added a table to illustrate the differences the rehabilitation exoskeletons from the aspects of mechanical type, type of actuator, control strategy, target parts of human, and validation methods.

Table I. Summary of Existing Exoskeletons and Their Related Technical Details

Devices	Mechanical Type	Actuator	Control Strategies	Target Parts of Human	Validation Methods
Lokomat [15]	Treadmill based exoskeleton	Motor	Position and impedance	Hip and knee	EMG activity
ALEX III [16]	Overground	Motor	Balance and impedance	Hip, knee and ankle	Reshape walking ability
ReWalk [17]	Overground	Motor	Force and impedance	Hip and knee	Walking assistance
Indergo [21]	Overground	Motor	Position and force control	Hip and knee	Walking assistance
Mindwalker [24]	Overground	SEA-motor	EEG-based position control	Hip, knee and ankle	Reshape walking ability
Ankle-foot Exoskeleton [31]	Overground	Pneumatic Muscle	Position and force control	Ankle	Reduction in Metabolic (21%)
Rex [37]	Overground	Motor	Balance and force control	Hip, knee and ankle	Reshape walking ability
HAL [38]	Overground	Motor	EMG-based force control	Hip, knee and ankle	Walking assistance

Author Response File: Author Response.docx

Round 2

Reviewer 2 Report

The authors have satisfactorily responded to all my questions.

Author Response

Thanks for the reviewer's patient instructions.

Reviewer 3 Report

Thank you for the resubmitted manuscript. The authors have improved parts of the manuscript. The new Table 1 is well received. However, there are still shortcomings in this manuscript. Unfortunately, several issues risen by the reviewers were not sufficiently addressed. Specifically, I am concerned about the choice of themes and the completeness of the reviewed work and exoskeleton-based systems.

The manuscript does not treat the selected themes in depth. The choice of themes (design, drive unit, intention perception, compliant control (what do you mean by "compliant"?) and efficiency evaluation seems rather random. How did you identify these themes?

In the manuscript, it is still not described how the reviewed literature was selected. Please describe the procedure.

It seems that there are many review-article about several of these themes already available. Please describe what perspective your review adds to the existing literature.

It seems that neither the referenced literature nor the mentioned systems are complete along with several of the themes. A short search on the Internet along the themes results in a large variety of articles that are not cited in your manuscript, e.g., https://www.sciencedirect.com/topics/nursing-and-health-professions/exoskeleton-rehabilitation, https://journals.sagepub.com/doi/10.1177/1729881417743554, just to mention some of the first search hits.

It seems that several terms are used in a misleading way. For instance, Line 185, the word "strategy" in the headline of Section 2.3: the following paragraphs describe diverse sensor-technologies (rather than strategies). Further, several terms are incomprehensible without a further context, such as "biological signal" or "mixed raw-machine hybrid signal" (Line 191). I really don't understand what you refer to.

Further, the list of technologies seems to be rather random and far from complete. There is a variety of review articles available that present these technologies in detail (e.g., see the literature about biomedical sensors in healthcare, biometry, etc.). There is a variety of technologies that could be suitable for use in exoskeletons that are not yet used in the existing systems.

Regarding the list of systems, it seems not complete. For example, EksoGT uses a hydraulic actuator. Further "Indergo" is misspelt, should be "Indego".

Please also note that the term "Mechanical Type" is incomprehensible or misleading.

Line 233: incomprehensible sentence; is there a ". " missing?

There is a large variety of acronyms in the paper, e.g., ZMP, EMG, EM, D-S, EEG, BP, ... Many of these are not defined. Please add a list of acronyms and abbreviations to increase the readability of the paper.

Note that there are exoskeletons for different purposes, also in healthcare and rehabilitation (some are for the entire body, some others are partial exoskeletons, e.g., only for leg, hand, etc.).

The part about evaluation (Section 2.5) is not informed by findings from the literature. Only the 6MWT or 10MWT are mentioned. There is a large corpus about testing such exoskeletons and their medical evaluation. The first hits of a search: https://jneuroengrehab.biomedcentral.com/articles/10.1186/s12984-020-00750-x, https://cjme.springeropen.com/articles/10.1186/s10033-019-0389-8. etc.

Regarding Section 3, it is still not clear how just these four "technical bottlenecks" were defined. The description of these bottlenecks is still very generic. Further, it should be mentioned how these bottlenecks could be addressed. Further, in which science areas would these be addressed; is it mechanical engineering, or material technology, or computer science, or medical engineering, or even others?

For several of the claims in Section 3, there is no proof given. Please give proof that these claims apply.

For example, you write that the suitability of the structure ... is significantly low. Why would this be so? Are there articles (or own research) that can give proof for this claim? further, writing that exoskeletons with more active DOF would be better suited, is not immediately evident; further, if this would be true, how would the mechanical layout of an exoskeleton look like.

Similar considerations can be made with the other three "bottlenecks".

Please also note that several sentences are incomprehensible; e.g., what is a "traditional sensor"? Why is a hysteresis important here? Do you think of specific sensors? why "human intention"? Note that diverse medical conditions might make it difficult to have a "one solution fits all"-type of solution. For example, one might want to measure the output of nerves, but in some conditions (e.g., stroke) this might be not suitable. In that case, a different solution, e.g., using eye movement might control the device.

Line 104ff: From the context, it would be expected that Reference 13 would point to the Five-Year Plans; however it points to an article about soft-skeletons and does not relate to the text. Further, when mentioning the Five-Year Plans, also the related plans in the US, in Europe (e.g., Horizon Europe), and other entities could be relevant to mention, as there are similar plans and cooperation agreements in-between them. In the review, this could then form a separate section about research policies within exoskeletons in rehabilitation.

Several of the references are not specified. E.g., what are references 7, 15, 16?

Author Response

Point 1: Thank you for the resubmitted manuscript. The authors have improved parts of the manuscript. The new Table 1 is well received. However, there are still shortcomings in this manuscript. Unfortunately, several issues risen by the reviewers were not sufficiently addressed. Specifically, I am concerned about the choice of themes and the completeness of the reviewed work and exoskeleton-based systems.

Response 1: We appreciate the reviewer’s critical comment about the completeness of this paper. We added the procedure of the selecting the reviewed literature (line 71 to line 78, in 3^rd paragraph, Section I). In Section I, we re-organized the structure of this paper and stated our considerations about why choose the theme and the relationships of the parts (please refer to the 4^th and 5^th paragraph in Section I).

“The exoskeleton systems are wearable human-machine devices made through anthropomorphic design and they can provide active assistance to the users according to their motion intention. It is one of the most potential technical researches to deal with the problems of disabled care and elderly assistance and rehabilitation [9]. The design of rehabilitation exoskeletons should match the ergonomic principles to guarantee the system can be corresponding with the distribution of human joints. Besides, to achieve rehabilitation procedures, the structure of the system should be specified including sensors, actuators, and controllers. Based on our surveyed literature, most exoskeletons were actuated by motors and some artificial actuators such as pneumatic muscles (PMs) [11], and shape memory alloy (SMA) [12] were reviewed. The control methods determined the performance of the exoskeletons conducting assistance and rehabilitation. The control algorithms were set to maintain human-machine interaction, and send the precise control commands to drive the actuators to perform corresponding auxiliary actions. Most interaction methods were achieved by feedback control based on detecting the information of the exoskeletons, the users, or the man-machine coupling system. Many behavioural and physiological sensors were introduced into the rehabilitation system to represent the status of the man-machine system. Whether it is helping patients with daily physiological activities such as walking, or performing regular rehabilitation training in accordance with the treatment plan, the exoskeleton system could easily and effectively complete the rehabilitation goals. Finally, it is necessary to evaluate the effectiveness of the exoskeleton systems and the performance of the assistance and rehabilitation.”

“The ergonomic design determined the matching performance of the exoskeletons which plays a fundamental role to achieve assistance and rehabilitation. The actuators, sensors, and controllers are the basic elements to conduct motion, perception, and control, which are the essential parts of the rehabilitation exoskeletons. The validation methods were designed to confirm the effectiveness of the rehabilitation procedure and the exoskeletons. In this paper, we reviewed the rehabilitation exoskeletons from the aspects of ergonomic design, actuation, perception, control and validation. We discussed the advantages and limitations of the man-machine interaction systems, and stated our considerations of designing and development of the rehabilitation exoskeletons in the future. We hope this paper can provide an overall guideline to design a rehabilitation exoskeleton system.”

Point 2: The manuscript does not treat the selected themes in depth. The choice of themes (design, drive unit, intention perception, compliant control (what do you mean by "compliant"?) and efficiency evaluation seems rather random. How did you identify these themes?

Response 2: The reviewer pointed that the manuscript should be discussed in depth. We appreciate the reviewer’s suggestion and accept this valuable suggestion. We revised the structure of the manuscript to be ergonomic design, actuation, perception, control, and validation methods. The proposed aspects were mentioned in many exoskeletons. We made it clear for the rehabilitation exoskeletons. We state the target of the five elements and specified the relationships of the five elements. Please refer to the highlighted parts in 4^th and 5^th paragraph in Section I.

Targets of the elements:

“The exoskeleton systems were wearable man-machine devices made through anthropomorphic design and they can provide active assistance to the users according to their motion intention. It is one of the most potential technical researches to deal with the problems of disabled care and elderly assistance and rehabilitation [9]. The design of rehabilitation exoskeletons should match the ergonomic principles to guarantee the system can be corresponding with the distribution of human joints. Besides, to achieve rehabilitation procedures, the structure of the system should be specified including sensors, actuators, and controllers. Based on our surveyed literature, most exoskeletons were actuated by motors and some artificial actuators such as pneumatic muscles (PMs) [11], and shape memory alloy (SMA) [12] were reviewed. The control methods determined the performance of the exoskeletons conducting assistance and rehabilitation. The control algorithms were set to maintain human-machine interaction, and send the precise control commands to drive the actuators to perform corresponding auxiliary actions. Most interaction methods were achieved by feedback control based on detecting the information of the exoskeletons, the users, or the man-machine coupling system. Many behavioural and physiological sensors were introduced into the rehabilitation system to represent the status of the man-machine system. Whether it is helping patients with daily physiological activities such as walking, or performing regular rehabilitation training in accordance with the treatment plan, the exoskeleton system could easily and effectively complete the rehabilitation goals. Finally, it is necessary to evaluate the effectiveness of the exoskeleton systems and the performance of the assistance and rehabilitation.”

The relationships of the elements:

For each aspect, we revise the paper and made a summary in the end of each aspect to identify the themes.

Design of rehabilitation exoskeletons:

“The wearable form of the rehabilitation exoskeletons guaranteed the exoskeletons can assist in ergonomic way. The exoskeletons must be designed based on the distribution of the humanoid characteristics such as muscle distribution, tendon-based transmission, and skeleton-based support. The position of the actuators should be placed along with human joints. The passive exoskeletons applied in military and industry fields showed great development in ergonomic design. Unlike the unpowered exoskeletons, the external energy should be introduced into the man-machine system because the rehabilitation exoskeletons are target to reshape or maintain the mobility of the people with moving dysfunction. Therefore, the rehabilitation exoskeletons should be powered exoskeletons including actuators, sensors, and control methods. In our surveyed literature, the motors were the most traditional actuators used in rehabilitation system, which have been developed for dacades. Some novel actuators were introduced into the rehabilitation exoskeletons such as PM and SMA inspired by their bionic characteristics.”

Actuation:

“The electric motors were still the most trustable actuators for the rehabilitation exoskeletons. Generally, the motors increase the torque through decreasing the rotation speed by the reduction boxes. The rehabilitation exoskeletons could provide powerful force to lift users’ body and keep balance. But the rehabilitation exoskeletons were of large volume because of the motors, reducers, and batteries, that made the system bulky and nonflexible. Therefore, there was a tradeoff between the flexible system and powerful output force. For the rehabilitation process, the movements of rehabilitation were achieved slowly to guarantee the safety during training. It implied that the motors were suit for assisting people with slow speed which was appropriate for the disabled and the elderly. The PMs were studied for decades from modelling, design, and control. The PMs were compliant actuators mimicking human muscles, which were lightweight, high power-to-weight ratio, and compliant. But there was an inevitable hysteresis between the inflation and deflation process. And it is hard to make the PMs system portable because the air pressure source cannot be portable. It means that the PMs system are similar to the platform-based rehabilitation exoskeletons driven by electric motors. The other novel actuators were proposed and applied into the rehabilitation system. The SMA-actuated soft exoskeletons achieved finger motions. Due to the limited scale of the output force, novel actuators were suitable to precise rehabilitation with small motion range. There is still a long road to transfer the novel actuators from lab to prototype.”

Motion intention perception:

“Human motion information had been drawing researchers’ attention from the aspects of the portable and wearable form, high accuracy, and low latency. Fortunately, the motion sensors can be manufactured into tiny type based the current semi-conductor development. But the accuracy of the motion recognition was still limited. The most popular biology information was represented by EEG and EMG depicting the command from brain and execution from muscles. But there were no non-invasive sensors can obtain the intention information with high signal-noise ratio. The invasive sensors showed great potentials in improving signal quality but it is still unclear whether the invasive sensors interfere human normal motion. However, the most efficiency sensors with high signal-noise ratio and low latency represented the motion behind the human, which introduced inevitable latency of the man-machine system. Motion prediction based on kinematic and dynamic information could solve the problem essentially. It is still feasible to use the current sensors to conduct perception during rehabilitation because the rehabilitation process was usually slow-speed and the latency of the sensors was acceptable in the slow rehabilitation process.”

Control methods:

“The interaction control theory has been developed for a long time and it has been rather mature for the rehabilitation exoskeletons during one rehabilitation process. It was not urgency to demand the real-time performance of the control methods because the rehabilitation process usually reduced to a slow process. But there was lacking rehabilitation strategy control which usually was made by the doctors. The rehabilitation strategy is to set the rehabilitation control with different groups of parameters related to users’ status. The optimization methods should be implemented to select the best parameters based on the judgement of the rehabilitation stage.”

Validation of the rehabilitation exoskeletons

“The validation methods for rehabilitation exoskeletons focuses on sensors used for biomechanics and energetics measurements. In general, kinematic and dynamic measurement was used to evaluate the flexibility of the rehabilitation exoskeletons and predicts energy expenditure indirectly. Metabolic cost measurement represented how much energy was saved by the rehabilitation exoskeletons. Muscle activity measurement indicates the fatigue of muscles, which was evaluated by EMG.

Kinematics describes the changes of displacement in linear or angular position and their derivatives, such as linear velocity, linear acceleration, angular velocity, and angular acceleration. And they can be obtained by the camera-based recognizing and wearable recognizing system. Kinetics aims to study forces that affect human motions. These forces can change the linear or angular motions. Force data can be obtained directly by using force and torque sensors. GRF is the representation of the human body’s impact on the ground measured by force plates, which can be used to analyse the force provided by exoskeletons. The energy for human motions comes from the chemical energy by digesting food and it flows in three directions: Entropy, Maintenance, and Muscle Energy. The energy of metabolic cost is part of Muscle Energy and it can be measured indirectly by recording respiratory flow, respiratory flow rate, heart rate, muscle activity, and etc.

However, the current evaluation methods were good at confirming the effectiveness of the exoskeletons but they were failed to confirm the effectiveness of the rehabilitation process. Clinically, doctors adjust the rehabilitation process based on the score-based evaluation methods which is the golden standard currently. The multi-information collected by the sensors should be introduced into the evaluation process to confirm the rehabilitation stage.”

Point 3: In the manuscript, it is still not described how the reviewed literature was selected. Please describe the procedure.

Response 3: The reviewer wondered the procedure of selecting the reviewed literature. We appreciate this valuable suggestion. We added a paragraph to show the procedure. Please refer to the highlighted part in 3^rd paragraph in Section I.

“To fully understand the efforts on rehabilitation exoskeletons, this paper reviews the published works on rehabilitation exoskeletons from 2003 to June 2021 in Web of Science database [5]. When the papers were reviewed, keywords ‘‘rehabilitation exoskeleton’’ is combined with “for the disabled’’ or “for the elderly” to collect the published literatures. The keywords search generated more than 38 journal and conference papers related to rehabilitation exoskeletons. Based on their contents, the research focuses of these papers can be categorized into three aspects: design, actuation, control, and validation methods.”

Point 4: It seems that there are many review-article about several of these themes already available. Please describe what perspective your review adds to the existing literature.

Response 4: The reviewer wondered the contribution of this manuscript. We appreciate the reviewer’s valuable comment. We revised the structure of the manuscript to be ergonomic design, actuation, perception, control, and validation methods. The proposed aspects were mentioned in many exoskeletons papers. We made it clear for the rehabilitation exoskeletons. We state the target of the five elements and specified the relationships of the five elements. Please refer to the highlighted parts in 4^th and 5^th paragraph in Section I.

Targets of the elements:

The relationships of the elements:

Point 5: It seems that neither the referenced literature nor the mentioned systems are complete along with several of the themes. A short search on the Internet along the themes results in a large variety of articles that are not cited in your manuscript, e.g., https://www.sciencedirect.com/topics/nursing-and-health-professions/exoskeleton-rehabilitation, https://journals.sagepub.com/doi/10.1177/1729881417743554, just to mention some of the first search hits.

Response 5: The reviewer proposed a hand exoskeleton named HEXORR in the exoskeleton book. We appreciate the reviewer’s valuable suggestion. In our manuscript, we have cited the hand rehabilitation exoskeleton actuated by shape memory alloy from journal paper.

The reviewer proposed a very valuable reference “Rupal B. S. et. al. Lower-limb exoskeletons: Research trends and regulatory guidelines in medical and non-medical applications, International Journal of Advanced Robotic Systems, vol. 14, no. 6, 2017.” In the suggested paper, large number of medical exoskeletons were reviewed based on the target part and composition. Compared to the proposed paper, we went further to discuss the exoskeleton systems based on the system structure. Besides, the advantages and limitations were stated in each parts.

Point 6: It seems that several terms are used in a misleading way. For instance, Line 185, the word "strategy" in the headline of Section 2.3: the following paragraphs describe diverse sensor-technologies (rather than strategies). Further, several terms are incomprehensible without a further context, such as "biological signal" or "mixed raw-machine hybrid signal" (Line 191). I really don't understand what you refer to.

Response 6: We appreciate the reviewer’s critical comment about the inconformity between the title and the content in Section 2.3. We revised the title to be “Motion Intention Perception” and the statement in Section 2.3.

We divided the perception methods into three groups: motion signal, biological signal and mixed man-machine hybrid signal. The biological signals included EEG, EMG, and etc.

“According to the difference in intention information acquisition mode, it can be divided into three intention perception methods based on motion signal, biological signal and mixed man-machine hybrid signal.”

Point 7: Further, the list of technologies seems to be rather random and far from complete. There is a variety of review articles available that present these technologies in detail (e.g., see the literature about biomedical sensors in healthcare, biometry, etc.). There is a variety of technologies that could be suitable for use in exoskeletons that are not yet used in the existing systems.

Response 7: We appreciate this valuable suggestion. We added a paragraph to show the procedure of selecting literature. Please refer to the highlighted part in 3^rd paragraph in Section I.

We revised the structure of the manuscript to be ergonomic design, actuation, perception, control, and validation methods. The proposed aspects were mentioned in many exoskeletons papers. We made it clear for the rehabilitation exoskeletons. We state the target of the five elements and specified the relationships of the five elements. Please refer to the highlighted parts in 4^th and 5^th paragraph in Section I.

Targets of the elements:

The relationships of the elements:

Point 8: Regarding the list of systems, it seems not complete. For example, EksoGT uses a hydraulic actuator. Further "Indergo" is misspelt, should be "Indego".

Response 8: We appreciate the reviewer’s critical comment. We revised “Indergo” to be “Indego”.

Point 9: Please also note that the term "Mechanical Type" is incomprehensible or misleading.

Response 9: We appreciate the reviewer’s suggestion. We revised the “Mechanical Type” to be “Working Type”.

Point 10: Line 233: incomprehensible sentence; is there a ". " missing?

Response 10: We appreciate the reviewer’s patient instruction and we checked all the English in the revised manuscript.

Point 11: There is a large variety of acronyms in the paper, e.g., ZMP, EMG, EM, D-S, EEG, BP, ... Many of these are not defined. Please add a list of acronyms and abbreviations to increase the readability of the paper.

Response 11: We appreciate the reviewer’s critical comment. We revised the acronyms and put them into a correct way.

Point 12: Note that there are exoskeletons for different purposes, also in healthcare and rehabilitation (some are for the entire body, some others are partial exoskeletons, e.g., only for leg, hand, etc.).

Response 12: The reviewer pointed out a good suggestion. We appreciate and accept the reviewer’s comment. We discussed the purpose of the exoskeletons (Please refer to the highlighted part. in Section 3).

Point 13: The part about evaluation (Section 2.5) is not informed by findings from the literature. Only the 6MWT or 10MWT are mentioned. There is a large corpus about testing such exoskeletons and their medical evaluation. The first hits of a search: https://jneuroengrehab.biomedcentral.com/articles/10.1186/s12984-020-00750-x, https://cjme.springeropen.com/articles/10.1186/s10033-019-0389-8. etc.

Response 13: The reviewer pointed that the evaluation was not intact. We appreciate the reviewer’s valuable suggestion and we added the validation methods. Please refer to the 2^nd and 3^rd, and 4^th paragraph in Section 2.5.

Point 14: Regarding Section 3, it is still not clear how just these four "technical bottlenecks" were defined. The description of these bottlenecks is still very generic. Further, it should be mentioned how these bottlenecks could be addressed. Further, in which science areas would these be addressed; is it mechanical engineering, or material technology, or computer science, or medical engineering, or even others?

Response 14: The reviewer wondered the statement of the technical bottlenecks of the rehabilitation exoskeletons. We revise the discussion and specified the technical bottlenecks from the aspects of design, perception, actuation, and control. The first sentences in paragraph 2, 3, 4 and 5 summarized the technical bottlenecks of the rehabilitation exoskeletons.

Ergonomic design:

“(1) The matching performance between the exoskeletons and human body is significantly low. The exoskeleton system with more active degrees of freedom has better flexibility, but complex structural composition and hardware lines affect the overall performance of the system. The joints of the human body and exoskeleton have obvious center misalignment during movement, which will reduce man-machine interaction. The coupling system may be deformed and misaligned during the interaction, which may reduce the power-assisted effect. Problems similar to the above are widespread in existing systems and severely disrupt ergonomics. To make the wearable exoskeletons more comfort, the unpowered exoskeletons inspired the novel design of rehabilitation exoskeletons. Besides, the actuators should be placed in corresponding with the distribution of human skeleton and muscles.”

Motion intention perception:

“(2) The recognition and prediction of human movement intention is not accurate enough. The motion intention is estimated by the mechanical signal obtained by the traditional sensor, although the result is reliable and stable, but there is a large hysteresis; the human intention based on biological signal analysis has good timeliness, but the data is unstable and the use process is cumbersome. Moreover, there were many kinds of existing data fusion algorithms, but motion mode recognition is often difficult to meet the requirements of safety and reliability accuracy, and methods with intent prediction capabilities are even rarer. The above difficulties are urgent problems to be solved in the human intention recognition and prediction. It is feasible to com-bine the physical model of human motion with the motion data to achieve fast and stable intention perception. The data is acquired by the body domain network including the human body movement, physical parameters and state. The data fusion algorithm combined with the human body kinematics and dynamics model should be studied.”

Actuation:

“(3) Lightweight and high power-weight ratio of driving units are difficult to achieve. Existing exoskeleton drive units have large volume and mass, and often lower power-weight ratios, resulting in large and bloated overall structural forms. Therefore, it is necessary to explore more effective driving forms as well as innovative design and optimization methods of high-power density driving units. Design permanent magnet servo motor are carried out by using conservative optimization design methods to realize the lightweight of permanent magnet servo motor. On this basis, implement multi-angle lightweight weight reduction design for servo motor selecting of new materials and new technology. The speed closed-loop control strategy and position closed-loop control strategy are designed to drive the permanent magnet motor to achieve high precision control. Realize the integral design including permanent mag-net servo motor body, brake, reducer, sensor and driver.”

Control:

“(4) The inconsistency between system motion control and human motion is prominent. The strong autonomy of human motion, the strong coupling of man-machine motion, and the complexity of the system model, have made it difficult for many control algorithms to achieve the goal of human-computer coordination and interaction. The above influencing factors should be considered to propose control strategies with more versatility and flexibility for the exoskeleton system. The exoskeleton system should meet the needs of the wearer to complete all kinds of basic movements and basic movement transformation. Dynamic control strategy should be implemented to the rehabilitation system and the stability should be confirmed based on real-time state detection and stability criteria. Finally, introducing adaptive control methods to ensure the reliability and consistency of the rehabilitation.”

Point 15: For several of the claims in Section 3, there is no proof given. Please give proof that these claims apply. For example, you write that the suitability of the structure ... is significantly low. Why would this be so? Are there articles (or own research) that can give proof for this claim? further, writing that exoskeletons with more active DOF would be better suited, is not immediately evident; further, if this would be true, how would the mechanical layout of an exoskeleton look like.

Response 15: We appreciate the reviewer’s critical comment about the inadequate discussion of the design. More evidence about the design should be added. We added the ergonomic design referring to the unpowered exoskeletons. The unpowered skeletons inspired the wearable design of the rehabilitation exoskeletons. Please refer to the highlighted parts in 1^st paragraph in Section 2.1. We proposed the possible solutions for the design of rehabilitation exoskeletons (please refer to the last two sentences in 2^nd paragraph in Section 3).

“The unpowered exoskeletons provided rich experience for the ergonomic design because the matching performance determined the distribution and transmission of the force [16]. There were already several representative passive exoskeletons, such as OX, UPRISE, Niudi, and FORTIS [16]. The OX was designed by Australian Government Department of Defense and it can transfer two thirds of the pressure borne by soldiers’ shoulder, spine, and legs to the ground. Mawashi Co. developed UPRISE transferring 50%-80% of the pressure borne by soldiers’ shoulder to the ground without interfering normal motion. UPRISE is constructed by using high-strength titanium alloy. Niudi Co., LTD from China proposed a modularized UE that can withstand 70 kilograms but weighs only 6 kilograms. The FORTIS is designed by Lockheed Martin Co. to help workers carry the heavy tools. The unpowered exoskeletons have great potentials in military, industry, rescue fields, and etc. The passive exoskeletons were well-bionic de-signed and constructed by dexterous structure. The ergonomic design of the passive exoskeleton could be referred to design the rehabilitation exoskeletons.”

“To make the wearable exoskeletons more comfort, the unpowered exoskeletons in-spired the novel design of rehabilitation exoskeletons. Besides, the actuators should be placed in corresponding with the distribution of human skeleton and muscles.”

Point 16: Similar considerations can be made with the other three "bottlenecks".

Response 16: We appreciate the reviewer’s critical comment about the inadequate discussion of the design. More evidence about the motion actuation, perception, and control should be added. We revised the manuscript and summarized the limitations at the end of each part. Please refer to the last paragraph in Section 2.2, last paragraph in Section 2.3, last paragraph in Section 2.4, and last paragraph in Section 2.5.

Design of rehabilitation exoskeletons:

“The wearable form of the rehabilitation exoskeletons guaranteed the exoskeletons can assist in ergonomic way. The exoskeletons must be designed based on the distribution of the humanoid characteristics such as muscle distribution, tendon-based transmission, and skeleton-based support. The position of the actuators should be placed along with human joints. The passive exoskeletons applied in military and industry fields showed great development in ergonomic design. Unlike the unpowered exoskeletons, the external energy should be introduced into the man-machine system because the rehabilitation exoskeletons are target to reshape or maintain the mobility of the people with moving dysfunction. Therefore, the rehabilitation exoskeletons should be powered exoskeletons including actuators, sensors, and control methods. In our surveyed literature, the motors were the most traditional actuators used in rehabilitation system, which have been developed for decades. Some novel actuators were introduced into the rehabilitation exoskeletons such as PM and SMA inspired by their bionic characteristics.”

Actuation:

Motion intention perception:

Control methods:

Validation of the rehabilitation exoskeletons

“However, the current evaluation methods were good at confirming the effectiveness of the exoskeletons but they were failed to confirm the effectiveness of the rehabilitation process. Clinically, doctors adjust the rehabilitation process based on the score-based evaluation methods which is the golden standard currently. The multi-information collected by the sensors should be introduced into the evaluation process to confirm the rehabilitation stage.”

Point 17: Please also note that several sentences are incomprehensible; e.g., what is a "traditional sensor"? Why is a hysteresis important here? Do you think of specific sensors? why "human intention"? Note that diverse medical conditions might make it difficult to have a "one solution fits all"-type of solution. For example, one might want to measure the output of nerves, but in some conditions (e.g., stroke) this might be not suitable. In that case, a different solution, e.g., using eye movement might control the device.

Response 17: We appreciate the reviewer’s critical suggestion. We revised the “traditional sensor” to be “sensors”. The hysteresis here is the latency of the signal transmission. We proposed the “hysteresis” to state the importance of the prediction of motion. Please refer to line 278 to line 281.

“However, the most efficiency sensors with high signal-noise ratio and low latency rep-resented the motion behind the human, which introduced inevitable latency of the man-machine system. Motion prediction based on kinematic and dynamic information could solve the problem essentially.”

Point 18: Line 104ff: From the context, it would be expected that Reference 13 would point to the Five-Year Plans; however it points to an article about soft-skeletons and does not relate to the text. Further, when mentioning the Five-Year Plans, also the related plans in the US, in Europe (e.g., Horizon Europe), and other entities could be relevant to mention, as there are similar plans and cooperation agreements in-between them. In the review, this could then form a separate section about research policies within exoskeletons in rehabilitation.

Response 18: We appreciate the reviewer’s critical comment. In this paper, we reviewed the main development of the rehabilitation exoskeletons from the aspects of design, actuation, perception, control, and validation. The Five-Year Plans was only one of the policies in China. We proposed the Five-Year Plans to show the government’s support and show the necessity of studying rehabilitation exoskeletons. In this manuscript, we just put the emphasises on the exoskeletons themselves. And it is impossible to collect all the policies supporting exoskeleton research.

Point 19: Several of the references are not specified. E.g., what are references 7, 15, 16?

Response 19: We appreciate the reviewer’s critical comment. We deleted the reference 7. Reference [15] ([17] in the revised manuscript) and [16] ([18] in the revised manuscript) are specified in the description in Figure 1.

Author Response File: Author Response.pdf

Round 3

Reviewer 3 Report

Thank you for the resubmitted manuscript. The authors have made further improvements to the paper. However, there are still some concerns about the structure of the paper. Further, it seems that there is an unspoken limitation in your selection of exoskeletons, as it seems that only exoskeletons supporting the lower limbs are included (in contrast to exoskeletons that support hand and arms). Also, the selection of technologies seems limited, as hydraulic exoskeletons (and maybe other technologies) still are not included. Please, describe such choices in the paper.

Quite much of the content in the review paper is rather vague. For each claim, please add literature references.

The authors have now described the procedure for retrieving the literature. However, there are many other review papers about exoskeletons available. The specific contribution of the paper remains still unclear. The five elements mentioned by the authors are not further outlined in the paper. These five elements should be used as a structuring element, also including reasoning why these five elements were chosen.

Regarding the validation (Section 2.5): Please describe more validation methods than the two mentioned in Ref 51. See also previous point 13. In the presented text, there are no more references given to what is described. In a review paper, please refer to the sources. Further, in this section, it is not clear what is going to be evaluated.

Line 359: minor issue: "and etc." --> please remove "and"

Table 1: "Working Type" is unclear. Also, it is unclear what "Overground" means, in contrast to what? Further, the validation methods mentioned seem to be application areas or rehabilitation goals, set aside the first and the sixth line.

lines 246ff: unclear

lines 265ff: unclear. this text mentions some methods and technologies without any context.

Section 3: This section does not contain a single reference. For all claims that you make, please add a reference. Further, much of this text is incomprehensible. What is the "matching performance"? Please describe this and also add a reference to prove this claim. The claim is also rather vague, as it is not clear what and where this presumably low performance appears. What do you mean by "distribution of human skeleton and muscles"?

You claim that the prediction [...] is not accurate. Could you add a reference or proof for this claim? Further, please explain the hysteresis. Why data fusion algorithms? Mode recognition, intent prediction, etc. are words that are not further explained; references are missing. (also ca line 390: something wrong with the sentence). Line 393: proof missing. What is the body domain network? Where and how is the data fusion algorithm applied?

Line 398: check drive units vs. driving units vs. driving forms. What are these? Is there proof for this claim? Why is it difficult to achieve? Line 400: Why "Therefore"? Line 402: incomprehensible sentence. lines 400-408: incomprehensible.

line 410: which inconsistency. Can you give proof for this claim? What is man-machine motion? lines 410-420: vague. diverse technologies, systems, strategies are mentioned without references to these; e.g., reference to dynamic control strategies or real-time state detection; etc.

Line 421ff: Vague. "Some breakthroughs": which ones? please give references. Which basic theories? Why theories and not proofs? Please specify theories, technologies, etc. What is a typical prototype?

line 433ff: very generic text that could fit at many places ... what is a drive-sensing-control-evaluation integrate exoskeleton? Line 436: why "revealed"? There is no proof for the claims in the last two paragraphs.

line 421: something is wrong with this sentence.

Regarding previous point 18: the diverse documents from the research agencies are usually publicly available. So, there is no reason why this should be impossible.

There are many minor issues, regarding language:

line 75: literature (not literatures)

line 77: of ???

line 83: guarantee that the system ...

line 84: Besides, ???

line 87: what are "artificial actuators"?

line 93: what are behavioural and physiological sensors?

line 127: Please define "unpowered exoskeletons"

line 218: what are "trustable actuators"? trust in the sense of what?

line 283: what is "slow rehabilitation process"?

line 275: something is wrong with this sentence.

line 324: rewrite: "it was not urgency to demand"

line 339: Who is "we"?

A list of abbreviations and acronyms is still not present.

Author Response

Point 1: Thank you for the resubmitted manuscript. The authors have made further improvements to the paper. However, there are still some concerns about the structure of the paper. Further, it seems that there is an unspoken limitation in your selection of exoskeletons, as it seems that only exoskeletons supporting the lower limbs are included (in contrast to exoskeletons that support hand and arms). Also, the selection of technologies seems limited, as hydraulic exoskeletons (and maybe other technologies) still are not included. Please, describe such choices in the paper. Response: The reviewer pointed that most of the reviewed exoskeletons are for lower limb and it is not comprehensive such as the upper exoskeletons and hands exoskeletons. We appreciate the reviewer’s valuable suggestion. We made it clear about the topic and revised the title of the paper to be “A Review on the Rehabilitation Exoskeletons for the Lower Limbs of the Elderly and the Disabled”. We clarified the target part of the rehabilitation exoskeletons for the elderly and the disabled in Section I. Please refer to the highlighted parts in abstract and last paragraph in Section I. The reviewer suggested that the hydraulic exoskeletons should be reviewed. We appreciate the reviewer’s valuable comment. We reviewed the hydraulic exoskeletons in Section 3. “Similar to the PM-actuated exoskeletons, the hydraulic exoskeletons depended on the pressure supplies [28]. The electrohydraulic actuator included motor, gear pump, and antagonistic installed cylinders [29]. The cylinders were controlled by servo valves and powered by combustion engines or electric motors.” Please refer to the highlighted parts in Section 3. Point 2: Quite much of the content in the review paper is rather vague. For each claim, please add literature references. Response: The reviewer suggested that each claim should be supported by the references. We appreciate this valuable comment. We re-organized the structure of the paper from the aspect of design, actuation, perception, control, and validation. In each part, we revised the paper and summarized the development of the rehabilitation exoskeletons based on the surveyed literatures. In the revised manuscript, 45 literature were added related to the rehabilitation exoskeletons for the lower limb of the elderly and disabled. Please refer to the highlighted parts in Section 2, 3, 4, 5, 6, and 7. Point 3: The authors have now described the procedure for retrieving the literature. However, there are many other review papers about exoskeletons available. The specific contribution of the paper remains still unclear. Response: The reviewer pointed that the references were not adequate and the five aspects should be stated further. We re-collected the literatures related to rehabilitation exoskeletons. “When the papers were reviewed, keywords ‘‘rehabilitation exoskeleton’’ and “lower limb exoskeleton” are combined with ‘‘for the disabled’’ or ‘‘for the elderly” to collect the published literatures. The keywords search generated more than 136 journal and conference papers related to rehabilitation exoskeletons. Papers not related to the research topic, repetitive articles, and articles related to walking aids for the blind and children’s rehabilitation aids were excluded. We selected 96 papers to review the exoskeletons from the point of manufacturing the rehabilitation exoskeletons. Based on their contents, the research focuses of these papers can be categorized into three five aspects: ergonomic design, actuation, perception, control, and validation methods.” Point 4: The five elements mentioned by the authors are not further outlined in the paper. These five elements should be used as a structuring element, also including reasoning why these five elements were chosen. Response: We re-organized the structure of the paper from the aspect of design, actuation, perception, control, and validation. In each part, we revised the paper and summarized the development of the rehabilitation exoskeletons based on the surveyed literature. Please refer to the highlighted parts in Section 2, 3, 4, 5, 6, and 7. We stated the reason why the five elements were selected. Please refer to the last paragraph in Section I. Point 5: Regarding the validation (Section 2.5): Please describe more validation methods than the two mentioned in Ref 51. See also previous point 13. In the presented text, there are no more references given to what is described. In a review paper, please refer to the sources. Further, in this section, it is not clear what is going to be evaluated. Response: The reviewer suggested that more validation methods should be introduced and the claim should be supported by the references. In the revised manuscript, we added 45 literature related to the rehabilitation exoskeletons for the lower limb of the elderly and disabled. Please refer to the highlighted part in Section 6. Point 6: Line 359: minor issue: "and etc." --> please remove "and" Table 1: "Working Type" is unclear. Also, it is unclear what "Overground" means, in contrast to what? Response: We appreciate the reviewer’s suggestion and we revised the manuscript. Point 7: Further, the validation methods mentioned seem to be application areas or rehabilitation goals, set aside the first and the sixth line. lines 246ff: unclear lines 265ff: unclear. this text mentions some methods and technologies without any context. Response: We revised the English of the paper. Lines 246ff is revised to be “The second method could effectively reduce the hysteresis of intention perception based on extracting and processing electroencephalogram (EEG) or myography data”. Lines 265ff is revised to be “For example, Northwestern Industrial University has achieved data fusion of EEG and electromyogram (EMG) through D-S (Dempster/Shafer) evidence theory and back propagation neural network [57].” Point 8: Section 3: This section does not contain a single reference. For all claims that you make, please add a reference. Further, much of this text is incomprehensible. What is the "matching performance"? Please describe this and also add a reference to prove this claim. The claim is also rather vague, as it is not clear what and where this presumably low performance appears. What do you mean by "distribution of human skeleton and muscles"? Response: The reviewer pointed that the Section 3 is not well-proofed. We appreciate the reviewer’s critical comments. We revised the DISCUSSIONS AND CONCLUSIONS from the aspects of ergonomic design, sensor-based perception, actuator, and control methods. We added the references to support our point of view. Please refer to the highlighted parts in Section 7. Point 9: You claim that the prediction [...] is not accurate. Could you add a reference or proof for this claim? Further, please explain the hysteresis. Response: The reviewer wondered the statement of prediction and it should be supported by literature. And the hysteresis should be defined clearly. We appreciate the reviewer’s valuable suggestion. We revised the statement of the prediction to be “Previous work has proposed the use of machine learning-based predictors based on EMG, kinetics, and kinematics to estimate the desired motion intention. More recently, several researches have explored using teleceptive sensing of terrain to improve prediction of desired locomotion [93] ”. And we defined the hysteresis of sensor-based motion perception. “There is a large hysteresis which is generated by the signal conversion and decoding process [89]. ” Please refer to the 3rd paragraph in Section 7. Point 10: Why data fusion algorithms? Mode recognition, intent prediction, etc. are words that are not further explained; references are missing. (also ca line 390: something wrong with the sentence). Response: The reviewer wondered the reason about data fusion. And supporting references should be added. Evidence has shown that, data fusion can improve the accuracy of motion perception and there were already many algorithms have been proposed. “Moreover, there were many kinds of existing data fusion algorithms such as radial basis function neural networks [90], convolutional neural networks [91], musculoskeletal model [92], etc, but motion mode recognition is often difficult to meet the requirements of safety and reliability accuracy. Previous work has proposed the use of machine learning-based predictors based on EMG, kinetics, and kinematics to estimate the desired motion intention. More recently, several researches have explored using teleceptive sensing of terrain to improve prediction of desired locomotion [93]” We revised the line 390 to be “Moreover, there were many kinds of existing data fusion algorithms such as radial basis function neural networks [90], convolutional neural networks [91], musculoskeletal model [92], etc, but motion mode recognition is often difficult to meet the requirements of safety and reliability accuracy.”. Point 11: Line 393: proof missing. What is the body domain network? Where and how is the data fusion algorithm applied? Response: The reviewer wondered the way of data fusion. Line 393: The musculoskeletal model provided a feasible way to combine the data and the human motion model [92]. The body domain network was designed to obtain the information of human body movement, physical parameters and state. “Sensor-based motion feedback is the basis of exoskeleton controlling and rehabilitation [55]. Moreover, there were many kinds of existing data fusion algorithms such as radial basis function neural networks [90], convolutional neural networks [91], musculoskeletal model [92], etc, but motion mode recognition is often difficult to meet the requirements of safety and reliability accuracy.” Point 12: Line 398: check drive units vs. driving units vs. driving forms. What are these? Is there proof for this claim? Why is it difficult to achieve? Line 400: Why "Therefore"? Line 402: incomprehensible sentence. lines 400-408: incomprehensible. Response: We appreciate the reviewer’s critical suggestion and we conducted English proofreading. We revised the Line 400, Line 402, and Line 400-408. Point 13: line 410: which inconsistency. Can you give proof for this claim? What is man-machine motion? Response: We revised the Line 410 to be “The deviation between system motion control and human motion is prominent. The strong autonomy of human motion, the strong coupling of man-machine interaction, and the complexity of the system model, have made it difficult for many control algorithms to achieve the goal of man-machine coordination and interaction. [55]” Point 14: lines 410-420: vague. diverse technologies, systems, strategies are mentioned without references to these; e.g., reference to dynamic control strategies or real-time state detection; etc. Response: We revised the Line 410-420 to be “The deviation between system motion control and human motion is prominent. The strong autonomy of human motion, the strong coupling of man-machine interaction, and the complexity of the system model, have made it difficult for many control algorithms to achieve the goal of man-machine coordination and interaction [55]. The exoskeleton system should meet the needs of the wearer to complete all kinds of basic movements and basic movement transformation. For the passive rehabilitation process, the trajectory-based control is enough to replay the predefined trajectory [96]. But the predefined trajectory is not suitable to different individuals. Introducing human motion intention input into man-machine interaction control was the effective methods to achieve more dexterous in assisting human motion [97]. Dynamic control strategy should be implemented to the rehabilitation system and the stability should be confirmed based on real-time state detection and stability criteria. Dynamic control involved dynamic modelling of the system, for example, simple mass-spring-damper model to characterize series elastic actuator [98], actuated dynamic model [99], and hybrid dynamic model [100]. Finally, introducing optimization control methods [101] to ensure the reliability and consistency of the rehabilitation.”. Point 15: Line 421ff: Vague. "Some breakthroughs": which ones? please give references. Which basic theories? Why theories and not proofs? Please specify theories, technologies, etc. What is a typical prototype? Response: We removed the vague statement and revised the prototypes to be products. Point 16: line 433ff: very generic text that could fit at many places ... what is a drive-sensing-control-evaluation integrate exoskeleton? Line 436: why "revealed"? There is no proof for the claims in the last two paragraphs. Response: We revised the last two paragraphs to be one. “A number of typical products such as Lokomat [17], Rewalk [19], HAL [40], etc, have been successfully developed, and application verification has been initially carried out. However, due to the difficulty of lightweight design, weak motion intention identification ability and poor motion control, it is difficult to obtain the qualitative efficiency to improve the existing exoskeleton assistance. It does not have the technical level of system lightweight, accurate identification and smooth motion, which restricts the promotion and application of such exoskeletons. Therefore, it is urgent to study high torque density motor lightweight driving system design theory and method. The multi-mode human movement biological information decoding and transmission mechanism should be revealed [93], and multi-source body and exoskeleton coordina-tion movement compliant control strategy needs to be established. And the final goal is to solve the key scientific problems in the engineering application of the exoskeleton robot for the elderly and the disabled, and provide the theoretical foundation and technical support for the development of wearable electromechanical systems.” Point 17: line 421: something is wrong with this sentence. Regarding previous point 18: the diverse documents from the research agencies are usually publicly available. So, there is no reason why this should be impossible. There are many minor issues, regarding language: Response: We revised the last two paragraphs to be one. “A number of typical products such as Lokomat [17], Rewalk [19], HAL [40], etc, have been successfully developed, and application verification has been initially carried out. However, due to the difficulty of lightweight design, weak motion intention identification ability and poor motion control, it is difficult to obtain the qualitative efficiency to improve the existing exoskeleton assistance. It does not have the technical level of system lightweight, accurate identification and smooth motion, which restricts the promotion and application of such exoskeletons. Therefore, it is urgent to study high torque density motor lightweight driving system design theory and method. The multi-mode human movement biological information decoding and transmission mechanism should be revealed [93], and multi-source body and exoskeleton coordina-tion movement compliant control strategy needs to be established. And the final goal is to solve the key scientific problems in the engineering application of the exoskeleton robot for the elderly and the disabled, and provide the theoretical foundation and technical support for the development of wearable electromechanical systems.” Point 18 : line 75: literature (not literatures) line 77: of ??? line 83: guarantee that the system ... line 84: Besides, ??? line 87: what are "artificial actuators"? line 218: what are "trustable actuators"? trust in the sense of what? line 283: what is "slow rehabilitation process"? line 275: something is wrong with this sentence. line 324: rewrite: "it was not urgency to demand" line 339: Who is "we"? A list of abbreviations and acronyms is still not present. Response: We appreciate the reviewer’s critical comment and revised the proposed English problems. Point 19 : line 93: what are behavioural and physiological sensors? line 127: Please define "unpowered exoskeletons" Response: We revised the definition of behavioural and physiological sensors and “unpowered exoskeletons”. “Many behavioural and physiological sensors were introduced into the rehabilitation system to represent the status of the man-machine system, where the behavioural and physiological sensors describe the kinematic features (such as joint angles, velocity, acceleration, etc) and human physical status (such as heart rate, Electromyogram (EMG), electroencephalogram (EEG), etc).” “The unpowered exoskeletons, which do not contain any powered elements (such as battery, electric motor, etc.) provided rich experience for the ergonomic design because the matching performance determined the distribution and transmission of the force [16].”

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